Amino acids in legumes

L-Canavanine (88), a nonprotein amino acid in legumes such as jack bean (Canavalia ensiformis (L.) DC.), has been shown to inhibit growth of susceptible plant species.53 L-3,4-Dihydroxyphenylalanine (l-DOPA, 89) has been isolated as an allelochemical from a cover crop, velvet bean (Mucuna pruriens (L.) DC. var. utilis).54... 

Wu, L., Guo, X., and Banuelos, G.S. 1997. Accumulation of seleno-amino acids in legume and grass plant species grown in selenium laden soil. Journal of Environmental Toxicology and Chemistry, 16(3) 491-7. 

The biological value of food proteins is greatly influenced by the ratio of their essential amino acids. Most food proteins, first of all plant proteins, are deficient in some essential amino acids. The limiting essential amino acid in legume and milk proteins is methionine, and in cereal proteins it is lysine. Beside the knowledge of the requirements of quality and composition of amino acids, it is equally important to know whether these amino acids can be best utilized as free amino acids, in the form of peptides or proteins. The following possibilities exist for improving the balance of essential amino acids in the proteins ... 

Rehr, S. S., E. A. Bell, D. H. Janzen, and P. P. Feeny, Insecticidal amino acids in legume seeds, Biochem. Syst., 1,63-67 (1973b). 

Methionine Animal proteins contain 2-4% of methionine, whereas plant proteins contain only 1-2% (average content is 1.7%). Methionine is the limiting amino acid in legumes. Methionine (and cysteine) is present only in small amounts in histones and is completely lacking in protamines. 

Glutamic acid is a component of many proteins, such as those in dairy products, meat, legumes, and mushrooms. However, only the free form of glutamic acid or glutamates has an effect on the glutamate receptors. When bound to other amino acids in a protein, it does not stimulate glutamate receptors. 

Characteristically, legume seeds are rich in protein and contain intermediate to high levels of lysine and threonine which are important in balancing the deficiencies of these essential amino acids in cereal diets. Certain legume proteins, such as soybean, also exhibit strong functional properties, especially water solubility, water and fat binding and emulsification. Thus soybean flours, protein concentrates and isolates have been used widely as nutritional supplements and functional ingredients in foods. 

Glutamic acid is the most abundant amino acid in the nervous tissue. In conventional proteins, both amino acids are usually found in larger quantities (especially in globulins) in cereal and legume proteins (18-40%). Wheat gluten (in its component gliadin) contains about 40%, soy protein contains about 18% and milk proteins contain about 22% of glutamic acid. 

From the nutritional point of view, denaturation is usually desirable since denatured proteins are more amenable to digestive enzymes than the native proteins. Denaturation, therefore, increases the bioavailabihty of proteins (e.g. sulfur-containing amino acids in cereals and legumes). Some antinutritional and natural toxic substances are also denatured, such as protease inhibitors (see Section 10.2.1.1), lectins (see Section 10.2.2.6), enzymes and other unwanted proteins and undesirable microorganisms. 

Pea protein is moderately deficient in the amino acids methionine and cystine, which are supplied in ample amounts by cereal proteins. However, the legume protein contains sufficient lysine to cover the deficiency of this amino acid in grain proteins. Hence, combinations of peas and cereal products supply higher quality protein than either food alone. 

Antibiotic effects of nonprotein amino acids have not been studied in much detail with soil microorganisms. The organs of legumes alone contain more than 300 nonproteinogenic amino acids, such as methylene-glutamine, common to all... 

The grain or pulse forms of legumes have a high total protein content (20-26%) and can therefore be used as a natural supplement to cereals. Pulses are normally deficient in the essential amino acids methionine and cystine but contain enough lysine, whereas cereals are deficient in lysine but contain enough methionine and cystine. 

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